Toposcopy has been shown to be a reliable clinical means to catheterize long, small diameter, and highly tortuous blood vessels, inaccessible by previous techniques. A toposcopic element everts from the tip of a conventional catheter. This extremely flexible polyurethane element has been fabricated in 3, 4 and 5 French sizes mated with 5, 6 and 7 French catheters, respectively, and is capable of eversion lengths in excess of 40\cm. The toposcopic catheter has been applied clinically for the local delivery of chemotherapy to brain tumors. Treatment is provided by positioning the conventional catheter in the internal carotid artery from a femoral entry; everting the toposcopic element through the carotid sinus, beyond the ophthalmic artery to avert retinal toxicity; and perfusing the tumor through the middle and/or anterior cerebral arteries. Not that a conventional catheter cannot safely negotiate the tortuosity of the carotid sinus. The catheter has been extensively used clinically in its present prototype form, and production techniques for its fabrication are being transferred to private industry to provide for a ready source of catheters for future studies.